1. Field of the Invention
The field of the invention is that of optical devices making it possible to measure the orientation of an object in space without contact. Diverse possible fields of application exist, but the main application is the detection of aircraft pilot helmet posture, thus making it possible to project in his visor an image superimposed exactly on the exterior landscape or to slave various systems of the craft to his gaze. The precision sought in such systems is of the order of a milliradian.
2. Description of the Prior Art
Various optical techniques making it possible to undertake on-helmet orientation measurement exist. Generally, spottable elements are installed on the helmet and are pinpointed by a system of cameras. The position of the images of these spottable elements makes it possible to determine the orientation of the helmet by calculation.
These elements may be passive or active. Passive elements are illuminated by an external source. To this end, retroreflecting cubic wedges may be used, which make it possible to reduce the problems of stray light, due to solar illumination. It suffices to dispose the optical emission and reception members on the same axis.
The active elements are generally light-emitting diodes. The cameras have a fixed fine-tuning distance and consequently a necessarily limited depth of field.
This technique presents a certain number of drawbacks. The quality of the image of each point imaged on the detector depends on the position of the helmet and its orientation, thus limiting the precision of the system if it is desired to cover a significant measurement volume or an appreciable range of rotation.